This invention relates to an easy-opening container packaging used in the industry of sealingly packing food.
Easy-opening container packagings tightly closed by heat seal and capable of being opened by ordinary human hand force are known.
This kind of easy-opening container packaging includes a type of sealed packaging having a seal provided between a cover member and a container formed of, for example, polyvinyl chloride, polyethylene, polyester, polypropylene, polycarbonate or the like. The seal is formed in such a manner that a hot melt bonding agent consisting of a mixture of, for example, an ethylene-vinyl acetate copolymer plastic, rosin, and a material such as polyterpene resin for providing adhesive properties or a petroleum wax, or a heat seal lacquer formed by dissolving in a suitable solvent a compound blended so as to have suitable peeling properties after heat sealing is coated to the cover base and is dried to form a film. Another type of sealed packaging is known in which a container formed of a suitable material is closed by heat sealing with a cover member to which an easy-peeling sealant film is bonded by a dry lamination method using a bonding agent. This easy-peeling sealant film is formed in such a manner that a sealant resin devised to be easy to peel off after heat sealing is converted to a film by an extrusion lamination or extrusion casting apparatus based on a method commonly used in this industrial field.
Techniques have been proposed which improve the heat seal strength to achieve a seal strength of 2.3 kg/15 mm or larger which satisfies the standard of retort food containers in conformity with the law (in Japan) while making the container easy to open.
For example, Japanese Patent Laid-Open No.62-251363 discloses a method in which an innermost layer plastic film of an easy-peeling sealant provided in association with an adjacent layer plastic is partially cut so that the innermost layer can be peeled from outer layers at the interface with the adjacent layer.
However, an easy-opening container based on this method has a layer-exfoliation structure such that the innermost layer of the easy-peeling sealant is peeled from the adjacent plastic film at the interface therebetween, so that the desired effects cannot be achieved unless the innermost layer plastic film is cut to a substantial extent, and cut waste pieces are formed when the innermost layer plastic film is cut, and there is therefore a risk of the waste pieces being mixed in the content food. It is difficult to completely remove such waste pieces.
Moreover, it is not possible to form a slit having a uniform thickness in a peripheral portion of the flange because of the non-uniformity of the thickness of the multilayer plastic sheet, resulting in dispersion of the peeling strength.
The strength of a heat seal between the cover member and the container of the ordinary easy-opening packaging is set to a value within a range of about 0.5 to 1.5 kg/15 mm such that the cover can be suitably separated by an ordinary human hand force.
Various techniques for increasing the heat seal strength above a certain level, e.g., 2.3 kg/15 mm which is the standard value of retort food containers in conformity with the law (in Japan) while achieving desired easy-opening properties are known which include methods relating to the compositions of the constituent materials and methods using the above described type of structure in which the innermost layer plastic film is cut. However, all the known methods are unsatisfactory because the heat sealing conditions are restricted or because the resulting sealing performance is inferior in stability. Also, in the conventional structures, the kinds of material and the use of materials are limited. That is, polypropylene, heat resistant polyester or the like cannot be used and only high density polyethylene can be used; the temperature at which the packaging is heated for retort sterilization is at most 120.degree. C. Further, because a particular means such as a notch in the flange is required, the width of the flange becomes unnecessarily large, which is undesirable in terms of appearance and cost. An ideal construction of easy-opening packaging is such that, for separation of the heat-sealed cover member from the container, the seal strength against an opening force applied from the outside of the flange is reduced to facilitate opening while the seal strength against a force applied from the inside of the flange or from the position of the content stuff is maximized for protection of the content stuff. In other words, it is necessary to increase the difference between the seal strengths with respect to the directions of application of opening forces from the inner and outer peripheries of the flange. Some of the conventional techniques are proposed in view of this point but they entail the above-mentioned drawbacks.